#define _CRT_SECURE_NO_WARNINGS 
#include <stdio.h>


int main() {
    int score = 0;
    scanf("%d", &score);
    if (score < 0)
        printf("F");
    else
        switch (score / 10) {
        case 10:
        case 9: {
            printf("A");
            break;
        }
        case 8: {
            printf("B");
            break;
        }
        case 7: {
            printf("C");
            break;
        }
        case 6: {
            printf("D");
            break;
        }
        case 5:
        case 4:
        case 3:
        case 2:
        case 1:
        case 0: {
            printf("E");
            break;
        }
        default:
            printf("F");
        }
    return 0;
}